Many devices that did not traditionally have communications capabilities are being replaced by updated devices that do have native communications capabilities. For example, newer, more expensive smoke detectors have native communications capabilities. However, this does not help with other smoke detectors and it is typically more cost effective to reuse the existing smoke detector and add in communications capabilities.
In adding such functionality, cost of components and assembly are a consideration. Another consideration is power consumption, as in a normal lifetime of smoke detector battery, only a very small portion of that lifetime is spent in an alarm activated state.